The present invention relates to the manufacture of a friction material by providing an open-celled foam preform and densifying the preform with carbonaceous material, and relates in particular to the manufacture of a friction material for aircraft brakes.
Carbon-carbon composites are widely used for aircraft brake friction materials. Carbon-carbon is attractive because it is lightweight and can operate at very high temperatures, and because it can, pound for pound, absorb a great deal of aircraft energy and convert it to heat. A major drawback with the use of carbon-carbon for this application is the high cost of raw material used to make the parts. Expensive carbon fiber is a significant component; sometimes up to 45% fiber is used in the composite. Fiber costs can often be the single largest contributor to the cost of making a friction material. Another drawback is that manufacture of carbon-carbon is a time-consuming process. The overall process for making a carbon brake disk is measured in weeks, and even months. Long cycle times are undesirable in a modern manufacturing environment. It is highly desirable to provide a process that has a reduced cost and shortened cycle time for making a carbon-carbon composite.
The inventions disclosed herein address those major drawbacks of manufacturing carbon-carbon composites: cost and cycle time. As pointed out by Hager and Lake (xe2x80x9cNovel Hybrid Composites Based on Carbon Foamsxe2x80x9d, Mat. Res. Soc. Symp. Proc., Vol. 270, (1992), pp. 29-33), it is possible to create a reticulated carbon foam structure from mesophase pitch. This structure would have substantial fiber-like properties. The foam could subsequently be used to reinforce or form a composite which would behave in many respects like a carbon-fiber reinforced composite. By using a foam preform, instead of carbon fiber, the lower cost fiber precursor can be used, and the preform can be made in a single foaming step, instead of using a laborious process of manufacturing a needled carbon fabric, and needling a preform, or making fiber prepregs which are compacted by subsequent molding.
The use of foam preforms for the manufacture of friction material is disclosed by Tsang et al. in U.S. Pat. No. 4,537,823. However, it is advantageous to: 1) use typically a graphitizable carbon foam from a mesophase pitch, rather than a glassy carbon foam, 2) fill the void spaces with a carbonaceous material, rather than a polymeric material or a slurry, and 3) provide foams with a pore size less than 500 xcexcm to facilitate subsequent densification.
Carbon foams made from mesophase pitch have been disclosed in Mehta et al., xe2x80x9cGraphitic Carbon Foams: Processing and Characterizationxe2x80x9d, American Carbon Society, 21st Biennial Conference on Carbon, Buffalo, N.Y., Jun. 13-18, 1993. These foams were not densified because the foams were to be used for lightweight structural applications that did not require densification.
Also, foams of carbonaceous material have been known and methods of preparing them have been disclosed for absorption or filtration media and supports for catalysts, etc., and is generally made from polymeric precursors (thermosets and thermoplastics) which usually produce amorphous or non-crystalline carbons.
The present invention comprises the use of precursors that can produce carbon foam preforms which result a reticulated structure having struts with fiber-like properties. The struts can be either crystalline, anisotropic, graphitizable carbons so that high strength and modulus, as found in current pitch-based fibers, can be reproduced within the strut regions (as defined below) of the carbonaceous foam, or isotropic nongraphitizable carbon. The solid strut regions within the foam could be tens to thousands of microns in length and have a diameter of tens of microns in width, leaving interconnected voids of tens to hundreds of microns in diameter so that high final bulk densities can be obtained after CVD or liquid phase densification. The thin characteristics of the strut regions within the foam will allow the crystallites within the mesophase precursor carbon to become preferrentially oriented along the axis of the strut mimicking the microstructures of carbon fibers. The bulk foam material can be controlled to provide either bulk isotropic or bulk anisotropic properties. Precursors include mesophase pitch, polyacrylonitrile (xe2x80x9cPANxe2x80x9d) and polyvinylchloride (xe2x80x9cPVCxe2x80x9d) as well as some resins such as phenolic and furfuryl alcohol. Pitch precursors undergo liquid-crystal (mesophase) formation during pyrolysis and result in a carbon with crystalline order. Currently, mesophase pitches are available that already have liquid crystal properties and provide an ideal precursor for foams. The term xe2x80x9cresinxe2x80x9d may be considered to encompass pitch when either pitch or resin is used as a precursor for carbonaceous materials. It is desired to produce foams with controllable pore structure which is interconnected (reticulated) so that it can be densified by either chemical vapor deposition (xe2x80x9cCVDxe2x80x9d), liquid phase densification processes such as Hot Isostatic Pressing (xe2x80x9cHIPxe2x80x9d), Pressurized Impregnation Carbonization (xe2x80x9cPICxe2x80x9d), Vacuum Pressure Infiltration (xe2x80x9cVPIxe2x80x9d), pitch or resin injection, or combinations of these densification processes.
The objectives of the disclosed inventions include:
(1) Production of a graphitizable reticulated foam preform in which the xe2x80x9cstrutxe2x80x9d structure mimics the properties of carbon fibers (to produce a direct substitute for carbon fiber preforms). Fiber-like properties are obtained within the strut members by use of a liquid crystal precursor (such as mesophase pitch) and strain action (both longitudinal and shear) occurring during the foaming process (alignment of the liquid crystals along the struts created during foaming and enhanced during subsequent heat treatment processing).
(2) Production of a non-graphitizable, reticulated foam preform structure and subsequently deposit graphitizable material around the strut members so as to mimic the properties of a fiber. The graphitizable material may be deposited by CVD or wetting of the strut surfaces by a liquid crystal material.
(3) Production of a foam preform with a reticulated structure capable of being further densified by the conventional processes discussed above. The porosity created by the reticulated structure allows the diffusion of gases or the infusion of liquids into the interior of the structure.
(4) Producing foam preforms suitable for manufacture of carbon-carbon composites used as friction materials in aircraft brakes, for thermal management, as well as structural applications. The foam preform is then subsequently densified with carbon or a carbonizable material or other fillers to enhance structural, thermal or tribological properties, and to produce a friction material, or thermal management material, or structural material. The combined composite should possess the structural, thermal and/or tribological properties required for friction materials, thermal management materials, and structural materials applications.
The disclosed inventions provide advantages over prior methods of making carbon-carbon composite friction materials:
(1) Mesophase pitch as well as other selected thermoplastic precursors produce high quality crystalline graphitizable carbons. In additon, the strut regions of the carbon foam provide a continuous network of fibrous reinforcement, as compared with discontinuous reinforcement found in fiber reinforced composites. Therefore, foam preforms should lead to improved thermal transport. Thermal transport is an important consideration in aircraft brake heat sinks and in thermal management materials.
(2) The foam preform approach is expected to be inherently less costly than the use of carbon fibers in carbon-carbon composites because less processing is needed.
(3) Near net shape forming of the final part may be possible with foams because they can be readily molded or extruded.
(4) The foam preform bulk properties are expected to be controllable and homogeneously isotropic or anisotropic. In addition, it is expected material property anisotropy may be controlled using processing variables or post foaming processes.
The present disclosure provides solutions to the above by comprising a process of manufacturing a carbon carbon composite material, comprising the steps of providing an open-celled carbon foam preform, and densifying the preform with carbonaceous material to provide the carbon-carbon composite material.